~ “ DL-Special DX ” Antenna ~

Two delta loops in phase (collinear)

ON6WG / F5VIF

Foreword

The purpose of this article is to propose an antenna with a high gain, a high efficiency, a very lowprice and that is easy to build for any frequency ( however there is one restriction as we will seefarther in the text ). Only wire, a few isolators and one connector will be required.

But at first, flashback to the nineties! We are in 1992 and a DXpedition eagerly awaited bynumerous amateur radio enthusiasts is announced. This is the FOØCI expedition and its activitywill be from Clipperton Island situated in the Pacific Ocean (Fig 1).

Fig 1 ( courtesy : C.Jost)

At that time there was a lack of directional antennas with gain at the ON6WG station and thecompetition to contact this DXpedition promised to be very arduous. So I had to find an antennasimple to build, quick to achieve and giving a substantial gain.

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The library of the ON6WG station contains some standard reference works as regards antennas.The idea to build this antenna came to me when consulting the inevitable book, “Low BandDxing” written by ON4UN and also another one “The Radio Amateur Antenna Handbook”written by W6SAI and W2LX. References of these works are in the heading titled “Appendix andBibliography” at the end of these pages.The result was equal to my expectations as it simply took me three calls through an indescribablepile up to contact FOØCI.Subsequently a number of other DX expeditions were contacted, again with the same ease. Thatconfirmed the excellence of the antenna.

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Fig 2 shows some of the DX stations or expeditions contacted with the DL-Special DX antenna. AVHF model was built to assess its performance before to build this antenna in different sizes towork on several HF bands. A description of the VHF model antenna was made at that time in the“Gigazette”, a magazine published by the Radio Club of Waterloo (Belgium). Following thisarticle I had very positive feedback. I still continue to use the DL-Special DX antenna from timeto time and it never ceases to amaze me with the results I get. However, only recently I decided todeliver a more detailed description in an article.

Introduction

The antenna presented here is consisting of two delta-loops placed one corner downward (like adouble V) and connected together in the same vertical plane (it is a phased array antenna). Thisscheme was chosen primarily for its ease of implementation and its ease of coupling to thetransmitter. Indeed, the DL-Special DX antenna has a radiation resistance close to 50 ohms andso, does not require an antenna coupler. If the antenna is made correctly the SWR will be closeto 1. However there is a restriction, it is a single-band antenna.This antenna made of wire can be achieved at a low cost price. On VHF bands or higher, its smalldimensions make it possible to build it on a frame (see Fig 3). One can even imagine to stick it ona glass window or on a bay window. When this antenna is built on a frame placed on a mast, itcan be oriented to different directions. On HF bands, even placed at a low height the lift-off angleof the wave is low enough to favor DX.

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Concerning the gain of the DL-Special DX antenna it is similar to that of a four elements Yagibeam. Fig 3 shows the VHF version of the DL-Special DX antenna in use at F5VIF.

Fig 3

Design and calculation

The antenna consists of two equilateral triangles placed one corner downward. The triangles arearranged in the same vertical plane and connected together in phase by the two top facing corners.This is also the connection point of the coaxial cable. So the feed system is very simple. Thediagram is shown in Fig 4. At resonance, the delta-loop length is 1.06 λ. The wavelength λ in feet is calculated from the following formula :

λ (feet) = 1005 / f (MHz)

The wavelength λ in meters is calculated from the following formula :

λ (meters) = 300 / f (MHz)

When building the antenna, cut the wire to 1.06 λ. The dimensions given above are valid for onedelta loop, obviously we have to multiply the result by two to make the DL-Special DX antenna.Keep the wire in one length. It is not necessary to cut the wire in the middle of the antenna toconnect the coaxial cable (see Fig 4). Then check the frequency which is obtained at theminimum SWR (this is the resonant frequency). Adjust the length if necessary.

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Fig 4

Feeding the antenna

This antenna design is symmetrical and normally the feed point requires a balun of 1/1 ratio. Inthis case, as the impedance is close to 50 ohms a 1/1 ratio choke balun can be added by placingferrite sleeves on the coaxial cable near the point of connection to the antenna (for moreinformations a paragraph is devoted to “Beads Baluns” in the “ARRL Antenna Book” 21st and22nd edition). This kind of balun, ready to use, can be bought also in any store specialized inradio amateur material.

Gain and radiation angle

Fig 5 shows the gain and the radiation pattern of the 145 MHz DL-Special DX model antennaplaced at one quarter wavelength above a soil of perfect conductivity. The values were obtainedby modeling using the “4nec2” program. The take off angle of the wave is at its lowest andtherefore favors long distance communications (DX). As it is shown in the diagram, the gainproduced is substantial and as discussed further in comparative tests, the antenna can standcomparison with a four elements Yagi.

Fig 6 shows the radiation pattern in the horizontal plane.

When the antenna is made to work on lower bands, Fig 7 and Fig 8 show similar radiationpatterns to the previous one (and here the antenna is also at a height of one quarter wavelengthabove a perfect ground). In this example it is the 28 MHz band which was selected. The radiationpattern is similar for all other HF bands.

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145 MHz

Fig 5

Fig 6

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28 MHz

Fig 7 DL-Special DX 0.25 λ above the ground

Fig 8

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Raising the height of the antenna over the ground will cause a change in the pattern radiation asshown below in Fig 9 and Fig 10.

Fig 9 DL-Special DX 0.5 λ above the ground

Fig 10 DL-Special DX 1 λ above the ground

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Fig 11 shows the impedance of the antenna at the resonnant frequency and on a selectedfrequency range between 28 MHz and 29 MHz. The point of resonnance is at the intersection ofthe resulting red line and the dashed line equal to 0.

Fig 11

Fig 12 shows the SWR on the DL-Special DX model antenna. The bandwidth is wide so theantenna can be used easily on the HF bands. A very low SWR can be obtained over the entireband.

Fig 12

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Note on the radiation pattern and the current distribution

Normally when using an equilateral triangle the feeder must be connected at a distance of 0.25 λof one of the corners in order to have an equal distribution of the currents through the delta loopand therefore a symmetrical radiation pattern. It is clear that in this design, this target cannot bereached. As an indication, Fig 13 exhibits this slight distortion in the radiation pattern when thefeeder is connected directly to the corner of the delta loop.

Fig 13

Referring to page 4, if you look closely at the pattern of the antenna in Fig 4 we see that thetriangles are not equilateral. The length of the perimeter of the triangles has not been modified butthe lower corners of the triangles are offset outwardly. However, even if we are in the case ofFig 13, the aperture angle of the antenna is very wide and on the HF bands the slight shift ofmaximum radiation will have little impact. Indeed it is a matter to gain S points in a specificdirection rather than in a specific part of the world. Note that this phenomenon is present on theVHF prototype.

Polarization

The DL-Special DX antenna offers dual polarization. In this model the dual polarization isdivided almost equally between horizontal and vertical polarization. This can prove to be a realasset to compensate for the fading on the HF bands. On the higher frequencies where horizontalor vertical polarization is specifically used it will allow to use one and the other without anyhandling. As an exemple, on the same frequency we can communicate via a relay using verticalpolarization while being in contact at the same time with another station which is using horizontalpolarization.

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Performance

As noted above, the gain of 11dBi is equivalent to the gain of a four elements Yagi antenna. Testsbetween the DL-Special DX and a 9dBd four elements Yagi have been done on 144 MHz onnumerous occasions to compare results obtained by diagrams with reports obtained duringcontacts. The antennas were placed at the same heigth on separate masts to avoid a possiblecoupling. In all cases with rare exception, during the tests in vertical or horizontal polarization, intransmission or reception, the reports are similar in both cases. Fig 14 and Fig 15 show the twoantennas used for the tests (here during a portable activity).

Fig 14 Fig 15

Conclusion

The DL-Special DX is a directional and mono-band antenna. Wire, some insulators and here is aspecial DX antenna that can be build for any band with a small budget. This antenna may beplaced at low heigth. The 50 ohms impedance is well adapted and a wide bandwidth with a SWRclose to 1 almost across the band gives to it a real ease of use. A wide opening angle allows tocover large areas with a significant gain. The hollow in the radiation pattern at the ends of theantenna is much less pronounced than for a dipole. This may lead to contacts in these directionsalso. Its construction made of wire gives to it lightness and resistance to bad weather. It can alsobe easily folded or rolled and only takes up little space. It will also arouse the interest andcuriosity of the stations who will contact you. So go off the beaten track and opt for its simplicityand originality. You will be amazed by the result !

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3D Radiation Pattern

Fig 16

The 3D view of the radiation pattern of the DL-Special DX antenna here placed one quarterwavelength above the ground gives a more concrete vision of the lobes as well as gain at thedifferent elevations.

Balun

Again, this antenna is belonging to the family of symmetrical antennas. The use of a 1/1 ratiobalun is therefore advisable (see page 4 § Feeding the antenna).

* * *

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9L1BTB DXpedition

Here is a practical example on how to fit a large size “DL-Special DX” on one mast only

The “DL-Special DX” antenna was used on15 meters with satisfactory results by Zbig,SP5BTB, during his Dxpedition to SierraLeone with the callsign 9L1BTB from 25of October till 11 of November 2013.On his web page there is a very nicepicture of the antenna. There is also adetailed and very interesting descriptionon how to build it and an easy andinspired idea to tune it. Just click on thelink below to reach the page.Congratulations, Zbig, for this nice job !

Link to 9L1BTB web page : …...... http://www.qsl.net/9l1btb

It is interesting to note the working conditions on my side during the QSO with 9L1BTB becausethis highlights the ability of the “DL-Special DX” in receiving signals. Thanks also to Zbig andhis skill in copying signals in a pile up. My sigs were certainly not among the strongest. Thepower used was 90 watts and the antenna was a center loaded mobile whip 1.6 meters in lengthfitted on a balcony at a height of twenty meters.

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Appendix

Note about the DL-Special DX antenna : ( I had to give it a name so that it is easy to find it onthe web) – means Delta Loop-Special (Special because this word is similar in many otherlanguages and because we are facing a very special antenna, and DX because the very low takeoff angle of the wave favors long haul communications). That's all !

Bibliography

The works below are part of the ON6WG library and were helpful in preparing this article. Theyrefer, among other things, to this type of antenna.

The radio Amateur Antenna HandbookWilliam I. Orr, W6SAIStuart D. Cowan, W2LX

Low Band DxingJohn Devoldere, ON4UN

Pictures and photographs Except for the image of Fig 1, all images are from the author. Fig 1 image : courtesy C.Jost (web)

To design and create field pattern diagrams the following software was used :

4nec2 …..........by Arie Voors …........... https://www.qsl.net/4nec2/

(Click on the blue link to reach the web page)

French translation of this article …. http://on6wg.pagesperso-orange.fr/Page%2014.html

ON6WG / F5VIF Web Site …........... http://pagesperso-orange.fr/on6wg

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